European Journal of Cardiovascular Medicine

Safe administration of anaesthesia, especially in paediatric patients, stands as a monumental achievement in medical science, with meticulous airway management taking precedence. ¹ Children's distinct anatomical and physiological characteristics set them apart from adults, necessitating specialized approaches in paediatric anaesthesia. Unlike adults, children possess funnel-shaped larynxes, and their airway capacity significantly differs. ² Thus, selecting the appropriate size of the endotracheal tube (ETT) becomes paramount in paediatric anaesthesia to mitigate potential complications such as inadequate ventilation or airway injury. ³

Traditionally, age-based formulas have been relied upon for ETT sizing, yet they come with inherent limitations, prompting the exploration of more precise methods. Ultrasound imaging has emerged as a promising tool for assessing airways, offering potential advantages over conventional approaches. Its non-invasive nature and ability to provide real-time visualization make it particularly appealing in paediatric cases where accurate airway assessment is crucial. ^4,5

This study aims to evaluate the utility of ultrasound in predicting ETT size in paediatric patients and compare it with age-based formulas. By doing so, we seek to ascertain the most accurate method for ETT sizing in this vulnerable population. Through meticulous examination and comparison of these methods, we aim to contribute to the refinement of paediatric anaesthesia practices, ultimately enhancing patient safety and outcomes. ^6,7

In pursuit of this goal, children under six years old undergoing general anaesthesia via an endotracheal tube were recruited for this study following informed consent. The study cohort underwent intubation using Cole's age-based formula, and the resulting leak was estimated at 10-20 cm water.⁸ Subsequently, ultrasound imaging was employed to calculate the subglottic diameter before intubation in the same patients. The correct ETT size used for each patient was then compared between the age-based formula and ultrasound methods to determine which approach yielded the most accurate tube size for paediatric patients undergoing anaesthesia. ⁹

Through this investigation, we endeavour to shed light on the efficacy of ultrasound in paediatric ETT sizing, potentially revolutionizing current practices and improving patient care. By harnessing the power of innovative technologies like ultrasound, we strive to enhance the precision and safety of paediatric anaesthesia, ensuring optimal outcomes for our young patients. ^10,11

This study was conducted in Umaid hospital and Mathura Das Mathur Hospital of Dr. S. N. Medical college and associated group of hospitals, Jodhpur after seeking clearance from Institutional ethical committee and obtaining written, informed consent from the patient's parents or relative.

Study design: Prospective study.¹²

Study period: From 15 September 2022 to 30 December 2022

ELIGIBILITY CRITERIA ^13,14

Inclusion criteria

1. Children with age less than 6 year
2. ASA grade 1 and 2
3. Children posted for elective surgery.
4. Children undergoing general anaesthesia with endotracheal intubation

Exclusion criteria ¹⁵

1. Difficult airway
2. General anesthesia with measures other than ET tube such as laryngeal mask airway
3. Parents refusing to give informed consent.
4. Patients having upper respiratory tract infection, tracheal and laryngeal pathology
5. Patient required Post operative mechanical ventilation.

Sample size calculation and statistical analysis.

The required sample size to show that the proportion of intubated correctly by the age based formula as compared to the ultrasound was found to be 63 children with 90% power and 5% level of significance with a difference of about 29% (difference between the two methods)

Proportion of successful intubation using Age based formula- 31/100

 Proportion of successful intubation using ultrasound- 60/100.

Following formula for single mean- paired t test has been used

 n= ( Z_1-a/2 +Z_1-b )² 2PQ /d²

 P= 31%+60% 2

 Q=100-P

 d= difference (29%)

Sample size was calculated to be 63 correctly predicted ETT size by ultrasound which was enhanced and round to 66 subjects.  

Preanesthetic assessment:  All patients examined during preoperative visit, prior to surgery by attending anaesthesiologist. In preoperative visit patient’s detailed history, general physical examination and systemic examination carried out. Basic demographic datas like name, age, sex, height, weight recorded. A detailed assessment of airway performed to rule out any airway difficulty. ETT tube size as per cole’s formula calculated and note. Routine investigations like hemoglobin (Hb), complete blood count (CBC), renal function test (RFT), urine complete, bleeding time (BT), clotting time (CT), or any specific test (if required) carried out. The patients kept nil per oral from 6 hrs prior to surgery for solid food, plain water  allowed till 2 hrs prior to surgery. ^16,17

Technology of ultrasonography and methodology

Standard anesthesia machine check protocol followed before each anesthetic.After shifting  the patient  to operation theatre, standard monitoring  attached. Baseline parameters like non- invasive blood pressure, heart rate, oxygen saturation (spo2), respiratory rate (RR) and electrocardiogram (ECG) noted.Calculated  volume of  intravenous fluid started through already secured iv line as per hospital protocol.  All patients premedicated with injections glycopyrrorlate 0.01 mg /kg and fentanyl 2 mcg/kg on table before induction. All patients were preoxygenated with 100% oxygen for three minutes. Anaesthetic induction agents were injection ketamine 2mg/kg or propofol 2mg/kg and inhalational sevoflurane with 100% oxygen.  Patients were allowed to breathe spontaneously through a face mask via Jackson Rees circuit with titrated doses of sevoflurane and 100% oxygen. A 5–13 Hz linear ultrasound probe   placed in the anterior region of the neck with the head in neutral position and the neck slightly extended. Scanning performed with B mode on, beginning along the midline from identifying the hyoid bone as a superficial, hyperechoic and curvilinear structure, with posterior acoustic shadowing in the transverse view. The probe was gently move caudally with a slight cephalad angle. The true vocal cords identified ultrasonographically as paired hyperechoic structures that move with respiration. These form the medial edge of the paired triangular hypoechoic vocal muscles. Then, the probe gently moved caudally to visualise the cricoid as a rounded, arch-like hypoechoic structure with hyperechoic edges.

Subglottic diameter measured ultrasonographically while mask ventilation is interrupted to minimise fluctuation in tracheal diameter. The transverse air column diameter measured at the level of cricoid cartilage. 3 readings were taken, and mean was calculated and noted.  All measurements performed by the same observer.If spo2 fall below 95% during measurement, ventilation supported with bag & mask. At first instance, endotracheal tube size chooses according to the preference of attending anaesthesiologist or measured by USG. The correct position of the endotracheal tube confirmed by capnography and by auscultation for bilateral breath sounds.Then, a standardised method for assessing leak performed to identify the correct size of endotracheal tube. The endotracheal tube size considered appropriate and clinically fit if there was an audible leak when the airway pressure increased to 15 cm of H2O. If the leak occured at less than 15 cm of H2O, the tube considers small and trachea will be reintubated with an ETT of size greater by 0.5 mm. If there is no audible leak till 25 cm of H2O, the tube inserted is considered large and trachea reintubated with an ETT of size smaller by 0.5 mm.

Following parameters noted: Transverse diameter at the level of cricoid as measured by ultrasonography (SD), inner diameter (ID) and outer diameter (OD) of the ETT finally placed which considered appropriate using leak test. Age, weight, height, endotracheal tube size and diameters measured by ultrasound was expressed as mean ± SD. Pearson’s correlation used to assess the correlation between age, measured diameters, and ETT outer diameter derived from the age-based formula with final appropriate size ETT outer diameter and proved statistically significance between two mehods by using McNemars test and dependent t- test ^18,19,20

Figure I. Ultrasound measurement of subglottic diameter. SM: Strap muscle, CC: Cricoids cartilage, R: Reverberation artifact, A and B: Air mucosal interface,AB: subglottic diameter

Table 1 - Internal and external diameter of the ETT followed with  subglottic diameter values.

Statistical methods:

 Mean and standard deviations were calculated for the agreement for correct ETT used with each of ultrasound ETT and age-based formula. ETT was assessed using Intra class correlation (ICC). Kappa value had been calculated to show the agreement of correctly and incorrectly identified measurements across ultrasound and age-based formula. Proportion of correct ETT used with the age-based formula and with that of ultrasound is compared using MacNemar’s test.

Total sixty-six children with age of 4 days to 72 months were recruited for this study after informed and written consent from the parents.  

AGE WISE DISTRIBUTION OF PATIENTS

Out of 66 children 18 children were ≤12 months of age, 16 children were between 13 to 24 months, 8 children were between 25 to 36 months, 12 children were between 37 to 48 months,10 children were between 49 to 60 months and 2 children were between 61 to 72 months of age at the time of procedure. Mean age was 30.30 months (SD 19.52) range 4 days to 72 months. Shown in table 1.

Table 1. Age wise distribution of patients (in months)

GENDER WISE DISTRIBUTION OF PATIENTS: Out of 66 children recruited, 50 (75.76%) were boys and 16 (24.24%) were girls (shown in table 2).

                               Table 2. Gender wise distribution of patients

Table 3. Weight wise distribution of patients in kg

WEIGHT WISE DISTRIBUTION OF PATIENTS

Mean weight of the 66 children recruited was 11.95 kg (SD 3.85) range (3.2 to 23.9).

Out of them 5 patients were ≤5 kg, 13 patients were 5.1 to 10 kg, 36 patients were 10.1 to 15,11 patients were 15.1 to 20 and 1 patient were ≥20.1 (shown in Table 3)

Table 4. Height wise distribution of patients

HEIGHT WISE DISTRIBUTION OF PATIENTS

Mean height of the 66 children recruited was 87.77 cm (SD 16.40) range (48 to 119 cm). Out of them 1 child was between 41to 50 cm, 3 children were between 51 to 60 cm, 5 were between 61 to 70 cm ,14 children  were 71 to 80cm , 10 children were 81 to 90 cm, 21 children were between 91 to 100cm, 9 patients were between 101 to 110 cm and 3 patients were between 111 to 120 cm  (shown in Table 4)

Table 5. ASA grade wise distribution of patients

ASA GRADE WISE DISTRIBUTION OF PATIENTS

As per the inclusion criteria, only ASA grade 1 and 2 patients were included for this study.

50 (75.76%) children were classified as ASA grade 1 while 16 children were ASA grade 2 (24.24%) (shown in Table 5)

Table 6 – Match between clinically used ETT size versus ETT size estimated by USG and ETT size calculated by age-based formula.

• In 9 (13.64%) patients clinically used ETT size were correct by age based formula.
• In 30 (45.45%) patients clinically used ETT size were correct by USG guided method.
• In 13 (19.70%) patients clinically used ETT size were correct by both age-based formula and USG guided method.
• In 14 (21.21%) patients clinically used ETT size were not correct by age-based formula and USG guided method.
• Clinically used ETT size matched with ETT size estimated by using USG method was in 43 patients (65.15%), whereas the ETT size calculated by age-based formula matched with clinically used ETT size in 22 (33.34%) patients. This difference was statistically significant (p <0.001 by using McNemars test) proving the superiority of ETT size estimated by USG guide method over age based formula.

  Table 6 - Pie diagram for the agreement of clinically used ETT size with USG guided method and age based formula shows that the ultrasound provided better prediction of ETT size in children (65.15 % of children) than age based formula that was correct in 33.34 % of the cases.

Table 7. Comparison of ETT size (in mm) estimated by USG and age based formula  with ETT size used clinically  on the operating table(OT) by dependent t- test.

• Mean of subglottic diameter estimated by USG(OD) was 6.24 mm (SD 0.88, range 4.5 to 8.8). As per the recommendations (ref) the USG tube size was calculated following the subglottic diameter estimation. Subglottic diameter considered as outer diameter of ETT and corresponding upto 0.5 mm bigger size outer diameter ETT were used then corresponding inner diameter was calculated.
• Mean of ETT size estimated by USG (ID) was 4.68mm (SD 0.59, range 3.5 to 6).
• Mean of ETT size calculated by age based formula (ID) was 4.45 mm (SD 0.61, range 3 to 5.5 mm).
• Mean of ETT size used clinically(ID) on OT table was 4.84 (SD 0.56, range 3.5 -5.5).
• Mean of the ETT size used clinically (OD) on OT table was 6.68 mm (SD 0.78, range 4.8 to 7.6) .
• Comparing the ETT size estimated by USG with the ETT used clinically showed significant difference ( t-value 3.91, p-value 0.0001).
• Comparing the ETT size estimated by age based formula with clinically used ETT size showed statistical significant difference (t-value 3.853, p-value  0002).

Table 7. Comparison of ETT size(in mm) estimated by USG and age based formula  with ETT size used clinically  on the operating table(OT) by dependent t- test.

Graph depicting mean values by, subglottic diameter, age based method , USG method and clinically used ETT ID and OD

TABLE 8: Gender wise calculation of mean of ETT used in male was 4.83 mm and in female 4.9 mm.

                  P value - 0.593 which was insignificant. Thus ETT size used was not affetcted by gender

TABLE 9:   ASA grade wise calculation of mean in grade 1 was 4.9 and in grade 2          was 4.68 and P-value 0.236 which was insignificant. Thus ETT size used was not affected by ASA grade

Intraclass correlation coefficient (ICC) was calculated to know the reliability agreement for the size of ETT which was clinically used for the patient with the size of ETT calculated using age-based formula and USG method. The agreement of ultrasound-based method with the clinically used ETT size was 0.902 was considered as excellent. It was also supported by a good 95% confidence limit. However, the same ICC for agreement for age-based formula method and clinically used ETT size was 0.841 which considered good. These values suggest that USG based methods is better than age-based formula in children.

TABLE 10:  Reliability Agreement of age-based formula and USG guided method to clinically used ETT size-

Graph 1 depicts that positive correlation between ETT size by age-based formula and    clinically used ETT size

Graph 2. Depicts that strongly positive correlation between ETT size estimated by USG and clinically used ETT size.

Endotracheal intubation remains a cornerstone of safe anesthesia induction, especially in paediatric patients. While age-based formulas have traditionally been used to estimate proper endotracheal tube (ETT) size, their accuracy is often debated. In contrast, ultrasound has emerged as a promising tool for ETT sizing due to its non-invasive nature and potential for real-time assessment. Our study aimed to evaluate the reliability of ultrasound-guided ETT sizing compared to age-based formulas in paediatric patients undergoing elective surgeries.

We found that ultrasound-guided ETT sizing demonstrated superior accuracy over age-based formulas, with statistically significant agreement between the clinically used ETT size and the size estimated by ultrasound. The mean subglottic diameter measured by ultrasound correlated well with the actual ETT size placed, emphasizing the efficacy of this method. Our findings align with previous studies, highlighting the potential of ultrasound as a reliable alternative for ETT sizing in paediatric anaesthesia.

Moreover, our study contributes to the growing body of evidence supporting the use of ultrasound in diverse populations, including Indian children. Ethnic variations in airway anatomy underscore the importance of locally validated methods for ETT sizing, further emphasizing the relevance of our findings. Overall, ultrasound-guided ETT sizing offers a safe, accurate, and ethnicity-specific approach to paediatric airway management, paving the way for improved patient outcomes in anesthesia practice.

Our study revealed a substantial difference in the accuracy of ETT size prediction between ultrasound and age-based formulas. Specifically, ultrasound correctly estimated the tube size in 65.15% of the paediatric population studied, whereas age-based formulas achieved correct predictions in only 33.34% of cases. This significant contrast underscores the superiority of ultrasound over traditional age-based methods for ETT sizing in paediatric patients.

The higher success rate of ultrasound in predicting the correct tube size can be attributed to its ability to provide real-time, precise measurements of airway dimensions. By directly visualizing the subglottic diameter, ultrasound allows for tailored ETT sizing based on individual anatomical variations, which may not be adequately accounted for by generic age-based formulas. Additionally, ultrasound offers the advantage of being non-invasive and operator-dependent, making it a practical and accessible tool for anesthesia providers.

Given these findings, we firmly conclude that ultrasonography represents a superior modality for selecting paediatric ETT size compared to age-based formulas. Incorporating ultrasound into routine clinical practice can lead to more accurate and personalized airway management strategies, ultimately enhancing patient safety and outcomes in paediatric anesthesia.

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